Method and device to control a positioning device for welding

ABSTRACT

In a method and device to control a positioning device, in particular a welding robot, for welding with an electrode holder and at least one force detection device to detect reaction forces at the electrode holder, a sum of reaction forces on the electrode holder; and the pose of the positioning device is regulated on the basis of the determined sum of reaction forces.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a method and a device to control a positioning device (in particular a welding robot) for welding with an electrode holder and at least one force detection device to detect reaction forces at the electrode holder.

2. Description of the Prior Art

In resistance spot welding (RSW), plates that are to be joined, for example, are pressed together by two welding electrodes and a welding current is conducted through the electrodes and the plates, wherein due to the increased transfer resistance between the plates their temperature is increased so much that the plates melt at that location.

If this is executed automatically by a positioning device (for example a welding robot), this device takes up predetermined poses in a position-regulated manner in order to position electrode holder and work piece(s) relative to one another. For this purpose the positioning device can move the work piece and/or the electrode holder. For example, a welding robot can apply a robot-guided electrode holder to a stationary work piece or, conversely, can supply a gripped work piece to a stationary electrode holder.

In the approach and/or in the welding pose, the electrode holder is closed and a welding spot is generated via movement of at least one electrode towards the other electrode. The poses to set the welding spots can, for example, be taught in advance via manual approach or be programmed offline and, for example, be taken up via proportional-integral-differential single joint regulators.

If in operation the position (i.e. bearing and/or orientation) of a work piece to be welded relative to a tool reference system of the positioning device—for example the TCP (“tool center point”)—of a welding robot now deviates from the position relative to which the pose was predetermined—for example since plates are deformed or imprecisely mounted in a feeder tool or feeder tool and positioning device are positioned imprecisely relative to one another—the position regulation attempts to forcibly reach the reference position. In particular given high strength and super high strength plates, this can lead to a degradation or a failure of the welding process, a damage to work piece, tool and/or positioning device and the like.

Therefore, in practice specific passive or active flexibilities—for instance, by what is known as a “Remote Center of Compliance” or electrode holders borne in a floating manner—have been provided that decouple the electrode holder from transversal forces during the welding process. In particular given non-stationary, directed electrode holders, to approach the next pose what is known as the holder compensation must be fixed in order to be able to position the electrode holder precisely. This entails difficult, complicated, power-consuming and error-prone mechanisms.

From EP 1 508 396 B1 it is known to regulate the contact pressure force of electrodes of a robot-guided electrode holder on the basis of forces that are detected by means of sensors arranged on the electrode holder without these having an influence on the robot pose.

From the German Patent Application of the assignee of the present application with the file number 10 2009 018 403.1 (not published before the priority date of the present application) it is known to flexibly regulate an axle of a welding robot so that the robot can evade (dodge) between work piece and electrode holder due to a contact force and thus can compensate position tolerances.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve automated welding.

The present invention uses an arrangement on the basic type known from EP 1 508 396 B1 cited above, for example, of force detection devices at an electrode holder to regulate the pose of a positioning device to position work piece and electrode holder relative to one another (in particular the pose of a welding robot) in addition or as an alternative to the holder adjuster. The regulation according to the invention is based on the insight that reaction forces are impressed on the element holder due to the pressing of the electrodes on the work piece(s) that is or, respectively, are to be welded, wherein the sum of reaction forces to generate the double-sided contact force increases. Since—given consideration of the reaction forces acting in total on the electrode holder—only those portions remain that result from a positional offset between work piece and closing point of the electrode holder, the pose of the positioning device can be regulated on the basis of the determined sum of reaction forces so that such an offset is compensated or at least reduced.

Both an individual forces and antiparallel force pairs (i.e. torques) or as well as components of these are generally designated as forces in the sense of the present invention for a more succinct presentation, such that a force sensor can also be designated as a moment sensor, for example to detect bending moments.

The result of two or more individual reaction forces or force components that are exerted on the electrode holder (in particular its electrodes) by a work piece to be welded is designated as a sum of reaction forces, for example that results on the welding electrodes tensioned opposite one another in the closing direction of the electrode holder and/or perpendicular to this. In consideration without algebraic signs (in terms of absolute values), the difference of two opposite reaction forces can also form a sum of reaction forces in the sense of the present invention.

According to a preferred embodiment, the sum of reaction forces is determined on the basis of a difference between reaction forces that act in total or in a predetermined direction (in particular the closing direction) on two electrodes of the electrode holder. A welding pose of the positioning device can then be regulated so that this is reduced, in particular minimized.

The sum of reaction forces can similarly be determined on the basis of forces that act between the electrode holder and its connection or, respectively, bearing, in particular an inertial bearing of a stationary electrode holder, or between a directed electrode holder and the positioning device bearing or guiding it. Reaction forces acting on a directed electrode holder generate as a result a corresponding force between the electrode holder and a positioning device guiding it, in particular its tool flange or electrode holder console. Reaction forces acting on a stationary electrode holder correspondingly generate as a result a corresponding force between the electrode holder and its inertial connection. Without positional offset, the inverse reaction forces that result from the double-sided, uniform contact pressure force mutually increase, such that no forces are transferred between the electrode holder and its respective connection. A welding pose of the positioning device can accordingly be regulated so that the force that acts between the electrode holder and its connection (in particular between the electrode holder and the positioning device) is reduced, in particular is minimized.

In the preceding explanation, weights of the electrode holder that also exert a force between the electrode holder and its connection (in particular between a directed electrode holder and a positioning device directing it) given the open electrode holder or the electrode holder closed in a correct position have not been taken into account; however, these are not reaction forces in the sense of technical mechanics. This allows these forces to be determined in advance and to be correspondingly taken into account (for example added or subtracted) in the determination of the sum of reaction forces.

According to a preferred embodiment, reaction forces that are used to determine the sum of reaction forces for the pose regulation at the positioning device are also used to determine and regulate the welding force or contact pressure force, i.e. that force with which the two electrodes are pressed against one another or on both sides against the work piece(s) that is or, respectively, are to be welded. For example, to determine the sum of reaction forces reaction forces on one or both electrodes of the electrode holder are determined; the actual applied welding force can also be determined from these and be regulated via comparison with a desired welding force.

The regulation of the positioning device advantageously ensues on the basis of the determined sum of reaction forces only in proximity to the pose to be occupied or given a pose already occupied, in particular while the electrode holder is closed or opened and/or as long as the electrode holder is closed.

For example, reaction forces can be determined by means of one or more pressure sensors (in particular piezosensors) and/or by means of one or more deformation sensors, in particular strain gauges. It is similarly possible to determine reaction forces by means of energy sensors (for example current sensors) that, for example, detect the power of actuators or, respectively, drive motors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: shows a welding robot with a controller according to one embodiment of the present invention.

FIG. 2: shows the opened electrode holder of the welding robot according to FIG. 1 in enlarged presentation upon approaching a work piece to be welded with positional offset.

FIG. 3 shows the electrode holder according to FIG. 2 in a closed state.

FIG. 4: shows the electrode holder according to FIG. 3 given a corrected pose of the robot.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a six-axle welding robot 1 with an electrode holder 2 (shown enlarged in FIG. 2 through 4). A controller 4 detects the joint or, respectively, motor angle q=(q1, . . . , q6) of the articulated arm robot 1, compares this—possibly after transformation to a Cartesian position r(q)—with a desired pose q_(s) or, respectively, desired position r_(s), and activates the drive motors of the robot 1 via a regulator (for example with PID individual joint regulators, a cascade regulation and/or current regulators), for example by means of specifying or impressing corresponding currents i.

The electrode holder 2 is shown enlarged in FIG. 4. It has a holder frame 2.1 made of aluminum and/or steel, connected with the robot 1; a holder arm 2.2 made of copper and arranged on the holder frame 2.1, on which holder arm 2.2 a stationary welding electrode 2.3 is arranged in turn; and a movable welding electrode 2.4 that can be moved and pressed against the stationary electrode 2.3 (as is in particular indicated by the double arrow in FIG. 3), for example by means of a schematically indicated hydraulic, pneumatic or electrical motor with spindle drive.

For a more compact depiction, four force sensors S1 through S4 are drawn together, wherein only the sensors S1 and S2, the sensors S1 and S3, the sensors S1 and S4 or only the sensor S4 are present, for example, according to preferred embodiments (as is explained in the following).

The first force sensor S1 is provided to detect a pressure force F1, F1′ or F1″ (see FIG. 2 through 4) of the movable electrode 2.4 on the holder frame 2.1, for example in the form of a piezosensor between the actuator of the movable electrode 2.4 and the holder frame 2.1, or in the form of strain gauges on the movable electrode 2.4 or its spindle. This pressure force can also similarly be determined, for example, from a power consumption of an electromotor to move the electrode 2.4.

The second force sensor S2 is provided to directly detect a pressure force F2 on the stationary electrode 2.3 and can, for example, be fashioned in the form of a piezosensor between this and the holder arm 2.2.

The third force sensor S3 is provided to indirectly detect this pressure force on the stationary electrode 2.3 and can, for example, be arranged in the form of a strain gauge arrangement on the holder arm 2.2 in order to detect bending moments or transversal forces F3 between holder arm 2.2 and holder frame 2.1, for example, from which the pressure force F2 on the stationary electrode 2.3 can be determined.

The fourth force sensor S4 is provided to detect a pressure and/or tensile force F4 between the holder frame 2.1 and the robot 1 or its tool flange or console, for example in the form of a piezosensor.

Shown in FIG. 2 is a situation in which two plates 3 that are to be spot welded with one another are offset relative to a reference configuration 3′ (indicated with dashed line), for example due to deformation, imprecise positioning or the like, when the welding robot 1 takes up the predetermined welding pose (see FIG. 1), i.e. positions its electrode holder 2 with regard to the reference configuration 3′.

The movable welding electrode 2.4, which is still inserted, thereby already contacts the offset plates 3. The sensors S1, S4 therefore detect a pressure force

S1: F1″, or, respectively,

S4: F4=F1″−G

with the weight component G of the electrode holder 2. If the electrode holder 2 is still closed and braced in this pose of the robot 1—in that the actuator of the movable electrode 2.4 exerts a contact pressure or welding force S on this—the plates 3 will deform as indicated in FIG. 3 (this is shown in an exaggerated manner in FIG. 3; in particular high strength and super high strength plates already counter high reaction forces given slight deformations). The force sensors then detect corresponding values

S1: F1′=c·Δ3+S;

S2: F2=S;

S3: F3=F2=S; and

S4: F4=c·Δ3−G

with stiffness c and deformation Δ3 of the plates 3.

Detection values F of the sensors provided in a preferred embodiment are respectively communicated to a regulator R of the controller 4. For example, this receives the difference (i.e. the sum, with algebraic sign, of the reaction forces on the two electrodes 2.3, 2.4):

$\begin{matrix} {{\Delta \; F} = {{{S\; 1} - {S\; 2}} = {{{S\; 1} - {S\; 3}} = {{{F\; 1^{''}} - 0} = {F\; 1^{''}}}}}} & \left( {{Fig}.\mspace{14mu} 2} \right) \\ {{or},{respectively},} & \; \\ {= {{{F\; 1^{\prime}} - {F\; 2}} = {c \cdot {\Delta 3}}}} & \left( {{Fig}.\mspace{14mu} 3} \right) \end{matrix}$

or the sum of the reaction forces exerted on the electrodes 2.3, 2.4, as detected by the force sensor S4, wherein the weight component G of the electrode holder 2 is hidden:

$\begin{matrix} {{F\; 4} = {F\; 1^{''}}} & \left( {{Fig}.\mspace{14mu} 2} \right) \\ {{or},{respectively},} & \; \\ {= {c \cdot {\Delta 3}}} & \left( {{Fig}.\mspace{14mu} 3} \right) \end{matrix}$

It is apparent that the welding force S is hidden by the determination of a sum or, respectively, results ΔF, F4 from reaction forces F1, F2 or F3, such that the remaining sum ΔF, F4 results purely from the offset relative to the reference welding position and this can therefore be corrected on the basis of this sum, for example in that a corresponding proportional allotment enters into the regulator R:

i=P1·(r _(s) −r)−P2·ΔF or, respectively,

i=P1·(r _(s) −r)−P2·F4

wherein this is simply based on a purely Cartesian proportional regulation for illustration. It is apparent that the pose of the robot 1 varies—i.e. the electrode holder 2 in FIG. 1 through 4 is raised (see FIG. 2, 3→FIG. 4 and the work piece plane drawn in a dash-dot line)—so that the sum ΔF or, respectively, F4 disappears. In spite of the offset of the work piece 3, this can be correctly welded in this manner with the welding force S exerted on both sides by the electrodes 2.3, 2.4.

As is apparent from the above explanation, sensor S4 between electrode holder 2 and robot 1 is sufficient for compensation of the offset. However, in a preferred development at least one of the sensors S1, S2 or S3 is also provided here in order to detect the welding force S and thus to be able to regulate via corresponding activation of the actuator of the movable welding electrode 2.4.

In a preferred embodiment, the regulation of the welding robot 1 that is explained in the preceding on the basis of the determined sum of reaction forces ensues only in proximity to the individual welding poses, where contact can occur between the welding electrodes 2.3, 2.4 and an offset work piece 3, while in-between these the robot 1 can, for example, occupy the next welding pose rigidly, precisely, in a position-regulated manner.

As indicated by the vector notation, the regulation can ensue in one or more directions of the components of the reaction forces, in particular in the closing direction of the electrode holder 2 and/or perpendicular to this.

Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art. 

1. A method to control a positioning device formed by a welding robot for welding with an electrode holder and at least one force detection device to detect reaction forces at the electrode holder, comprising the steps of: in a processor, determine a sum of reaction forces on the electrode holder; and from said processor regulate the pose of the positioning device on the basis of the determined sum of reaction forces.
 2. A method as claimed in claim 1, comprising determining the sum of reaction forces from differences between reaction forces that act on two electrodes of the electrode holder.
 3. A method as claimed in claim 1, comprising determining the sum of reaction forces from a force that acts between the electrode holder and a bearing of the electrode holder.
 4. A method as claimed in claim 2, comprising regulating a pose of the positioning device to reduce the difference between at least one of reaction forces and the force that acts between the electrode holder and the bearing.
 5. A method as claimed in claim 1, comprising determining the sum of reaction forces from a force that acts between the electrode holder and a bearing of the positioning device.
 6. A method as claimed in claim 5, comprising regulating a pose of the positioning device to reduce the difference between at least one of reaction forces and the force that acts between the positioning device holder and the bearing.
 7. A method as claimed in claim 1, comprising determining and regulating a welding force.
 8. A method as claimed in claim 7, comprising determining the welding force from reaction forces that are also determined for determining the sum of reaction forces.
 9. A method as claimed in claim 1, comprising determining reaction forces using at least one pressure sensor selected from the group consisting of piezosensors, deformation sensors, strain gauges, energy sensors and current sensors.
 10. A control device to control a positioning device formed by a welding robot for welding with an electrode holder and at least one force detection device to detect reaction forces at the electrode holder, said control device comprising: a processor configured to determine a sum of reaction forces on the electrode holder; and said processor being configured to regulate the pose of the positioning device on the basis of the determined sum of reaction forces.
 11. A non-transitory computer-readable storage medium encoded with programming instructions, said storage medium being loaded into a computerized control device to control a positioning device formed by a welding robot for welding with an electrode holder and having at least one force detection device to detect reaction forces at the electrode holder, said programming instructions causing said computerized control device to: determine a sum of reaction forces on the electrode holder; and regulate the pose of the positioning device on the basis of the determined sum of reaction forces. 